Incremental optical motion encoders are used to resolve the position and movement of an object along a particular route. Such encoders generally include a light source for emitting a light beam, light modulation means for modulating the light beam in response to movement of the object along the route, and detection means for receiving modulated light and for producing discrete electrical signals representing detection of light by the detectors. As the light is modulated in response to the movement of the object, the stream of electrical signals from the detector assembly produces a continuous wave form usually resembling a square wave. The position of the object along its route determines the position of each signal in the wave form. The phase of the wave form differs depending upon the location of the object. Thus, signals from the detectors can be used to indicate a change in location of the object along the route. Two or more out-of-phase signals from separate detectors can be used to detect both change in location and change of direction of movement.
For an incremental motion encoder to indicate the absolute position or location of the object along its route, an index pulse is generated at least once along the route. The incremental signals can be used to count incremental movement from the index pulse. If the position of the object is known at the time the impulse pulse is generated, the absolute position of the object at any place along the route can be determined.
Therefore, to provide an indication of absolute position, change in location and direction of movement, an incremental encoder usually requires three channels of information. Two channels are derived from two or more out-of-phase encoder signals that are produced throughout the route of the object, and the third is an index signal produced at least once along the route at a known position of the object.
In a typical embodiment, such a position encoder or movement detector is used to measure the angular position of a shaft. Depending on the use of such a shaft angle encoder, a high degree of resolution and accuracy may be needed; for example, in automotive crankshaft angle measurement applications or accelerator speed control, a resolution of 2000 increments per revolution of the shaft may be necessary. Accuracy of the correlation between the signal from the encoder and the actual mechanical position of the shaft is also important. Mechanical alignment discrepancies can adversely affect accuracy as can electrical noise, due to the very small dimensions of the code wheel, the transmissive sections and the non-transmissive sections.
To accurately detect the index pulse, a push-pull electronic arrangement may be used to determine the location of the index pulse. In such an arrangement, two photodetectors are arranged laterally adjacent one another in vertical alignment with a light source; a spoked circular code wheel fixed to the shaft separates the light source and the detectors. When one of the detectors is illuminated, a logic signal of one sense, such as 1, is produced. When a spoke on the code wheel occults the illumination, the opposite logic signal is produced, such as a logical 0. When the index window passes over the detectors, one detector will generate a logic 1 of long duration, followed an instant later by a logic 1 from the second detector. The direction of travel of the code wheel may be determined by sensing which detector is first to generate a long-duration logic 1.
Another desirable feature of code wheel encoders is to determine the index location regardless of the direction of rotation of the code wheel or shaft. Since code wheels ordinarily can rotate both clockwise and counter-clockwise, it is desirable that the encoder apparatus produce an index pulse output at a consistent logic level regardless of the direction of rotation. In conventional code wheel encoders employing push-pull optics and electronics using two adjacent light sources and two adjacent detectors, the logical value of the index pulse is different depending on the direction of rotation. Thus, if the wheel is rotating clockwise, such conventional circuits will produce a low logic value index pulse, and when the code wheel is rotating counter-clockwise, the conventional circuit will produce a high-logic value index pulse. This feature is undesirable, because the circuitry receiving the index pulse must accommodate for both high and low logic values. Such accommodation ordinarily requires additional undesirable hardware.
Thus, it is desirable to provide an optical encoder apparatus which produces an index pulse having the same logic value regardless of the direction of rotation of the code wheel. Preferably, this is provided by modifying the photo receptor or photodetector assembly rather than changing external hardware, and using a conventional push-pull encoder.